Alkali metal borohydride concentrate

ABSTRACT

Alkali metal borohydride concentrates are prepared by dry blending alkali metal borohydride powder with a dry non-reactive plastic resin, melting and extruding the blended mixture to form an encapsulated alkali metal borohydride concentrate, and then chopping the extrusion into pellets. The pellet concentrate may then be blended into other materials.

This application is a division of application Ser. No. 272,867, filedJune 12, 1981, and now abandoned.

BACKGROUND OF THE INVENTION

The use of alkali metal borohydrides such as sodium borohydride andpotassium borohydride as blowing agents in connection with theproduction of cellular articles is well known. For example, U.S. Pat.No. 2,951,819 utilizes an alkali metal borohydride in connection withthe manufacture of cellular articles. U.S. Pat. No. 3,167,520 relates tocellular articles prepared from polystyrene by extruding polystyreneresin containing an alkali metal borohydride which decomposes duringextrusion to form a cellular article. The invention of this applicationis directed to an improvement to the technique of aforesaid U.S. Pat.No. 3,167,520.

SUMMARY OF THE INVENTION

The invention involves the use of alkali metal borohydrides such assodium and potassium borohydride as blowing agents useful in connectionwith the production of articles having a foamed or cellular structure.One particularly advantageous embodiment of the invention involves theuse of a stable borohydride concentrate mixture containing from about0.5% to 20% by weight of an alkali metal borohydride with a dry inertresinous polymer. The concentrate should contain no more than about 0.1%adsorbed water or the borohydride will hydrolyze. Such water may beadsorbed on the surface of the borohydride and/or on the surface of theresinous polymer.

The concentrates of the invention may be made by dry blending from 0.5%to 20% by weight of powdered alkali metal borohydride with a dry inertresinous polymer to form a mixture containing less than about 0.1% byweight water; heating the mixture to a temperature above that of themelting point of the resin to form a molten mixture; extruding themolten mixture and then cooling the extrusion to form a solid, stableconcentrate; and then forming the cooled extrusioned concentrate intopellets by conventional techniques such as chopping, grinding and thelike.

Another aspect of the invention comprises a method for making cellulararticles by contacting a mixture of alkali metal borohydride and aresinous polymer with a high surface area silica activating agent havinga proton source adsorbed on its surface so as to cause said alkali metalborohydride to hydrolyze thereby causing said resinous polymer to have afoamed cellular structure. The alkali metal borohydride may be in theform of the concentrate described above or as neat powdered alkali metalborohydride.

DETAILED DESCRIPTION OF THE INVENTION

The concentrates of the invention may be used to incorporate low levelsof alkali metal borohydride into the same or different resins with theobject of using the incorporated borohydride to foam the resin where anadditional protonic activator such as water, alcohol or acid is alsoadded to the resin to react with the borohydride to produce hydrogen.

When used as a blowing agent for thermoplastic resins, up to 0.5% NaBH₄may be incorporated into the resin from the concentrate. The preferredrange is up to 0.25%.

The use of concentrates to incorporate low levels of borohydride intoresins has several advantages. First of all, the concentrates protectthe borohydride from adventitious hydrolysis during handling and may bestored for periods on the order of a year without the occurrence ofsignificant hydrolysis. Secondly, it is much easier to incorporate theborohydride at very low levels such as less than 1 wt.% into the resinby using the concentrates. Thirdly, overall handling such as weighingand making uniform mixtures is much easier using the concentrates thanusing neat borohydride. Because such highly reactive material has beenconcentrated and rendered substantially inert, employee safety isenhanced. In addition, very uniform dispersions of borohydride in theresin are obtained using the concentrates.

The stable borohydride concentrate of the invention comprises from about0.5% to 20% by weight of an alkali metal borohydride, less than about0.1% adsorbed water, balance essentially being dry inert resinouspolymer. The 0.5% lower limit for borohydrides was selected primarilybecause lower concentrations would necessarily involve the use ofexcessive amounts of the concentrate and the 20% upper limit wasselected because higher levels would require excessively small amountsof the concentrate and thereby hamper the attainment of uniformdispersion in the thermoplastic polymer to be foamed. It is preferred toinclude the alkali metal borohydride in amounts between 2 and 10%because such range permits let-down ratios on the order of about 20:1 asare normally desired in the industry. Sodium and potassium borohydridesare contemplated for use in connection with the invention. Theconcentrate should contain no more than about 0.1% by weight of adsorbedwater, preferably not more than about 0.01%, in order to preventhydrolysis of the borohydride. Thus, the borohydride and resin must bevery dry when mixed to form the concentrate and moisture pick-up shouldbe avoided during such mixing.

The inert resinous polymer component of the concentrate comprises resinsin which reactive protonic hydrogen atoms are substantially absent. Suchresins include polyolefins, polydienes, polystyrene,polyphenyleneoxide-styrene, polyacrylates, polyvinylchloride,polyvinylacetate and the like.

The alkali metal borohydride concentrates of the invention are preparedby dry blending from about 0.5% to 20% by weight of the borohydride withthe inert resinous polymer to form a dry mixture thereof. The mixtureshould contain less than about 0.1% of adsorbed water. The mixture isthen treated to a temperature in excess of the melting point of theresin, extruded, cooled, and formed into pellets.

As mentioned previously, the alkali metal borohydride has utility as ablowing agent in connection with the manufacture of cellular or foamedresinous articles. Typical resinous polymers that may be foamed with useof the concentrate include but are not limited to polyolefins,polydienes, polystyrene, polyphenyleneoxide-styrene, polyacryletes,polyvinylchloride, polyvinylacetate and the like.

Cellular articles may be manufactured by mixing the alkali metalborohydride concentrate mentioned above with a resinous polymer and aneffective amount of an activating agent capable of donating an acidicproton to the alkali metal borohydride so as to hydrolyze substantiallyall of the alkali metal borohydride while the resinous polymer is moltenand thus form a cellular article. The activating agent may comprise afinely divided high surface area solid having a proton source adsorbedon its surface. A preferred activating agent is finely divided highsurface area silica.

An example of a form of silica suitable for use in the invention is thatmanufactured by Cabot Corporation under the designation "CAB-O-SIL".Such type of silica is manufactured by fuming. It is preferred toutilize silica having a surface pH of less than 7 because acidicmaterials aid in the decomposition of alkali metal borohydrides. It isespecially preferred to have a pH in the range of 4 or lower. Finelydivided, high surface area activating agents function to providenucleation sites for foam to form. These materials have a very highcapacity for adsorbing liquids such as water and yet remain afree-flowing solid so as to thereby aid in the promotion of uniformityduring the blending or mixing stage of the process.

Suitable proton sources include water, mineral acids, carboxylic acids,alcohols, sulfonic acids, polyacrylic acid, polystyrene sulfonic acidand like materials. Water is preferred due to its relative cost,availability, and low corrosivity.

It has also been discovered that the silica activators described abovemay also be used in combination with alkali metal borohydride materialsother than the concentrate described above. For example, powdered alkalimetal borohydrides may be utilized in combination with the silicaactivators to produce foamed polymers. As in the case of the concentrateadditions, the alkali metal borohydride and a resinous polymer arecontacted with a sufficient amount of activating agent to causesubstantially all of the borohydrides to hydrolyze.

The following examples further illustrate the practice of the invention.

EXAMPLE 1

Sodium borohydride was concentrated in polypropylene by the followingprocedure. Ninety parts by weight of previously dried, powdered,unstabilized polypropylene resin (melt index 12) and 10 parts by weightof NaBH₄ that was previously dried 24 hrs at 100° C., were thoroughlymixed in a dry blender. The resultant mixture was extruded through a 1/8inch diameter die at about 450° F. The extruded rod was pelletized inline with the extruder. The extruded concentrate was examined with a lowpower microscope and an extremely uniform dispersion of minuteborohydride particles was found. Virtually no gas bubbles were observedin the resin pellets.

EXAMPLE 2

Sodium borohydride was concentrated in NORYL resins in the followingmanner. Ninety parts by weight of dry NORYL resin FN 215, a proprietaryGeneral Electric Co. blend of polyphenylene oxide and polystyrene inpellet form, was mixed with 0.1 part by weight of mineral oil and 10parts by weight of NaBH₄ powder, that was previously dried at 100° C.for 8 hrs. The resultant dry blended mixture was then extruded at about450° F. and pelletized as described in the previous example. Theconcentrate pellets were equivalent to the pellets of Example 1.

EXAMPLE 3

Sodium borohydride was concentrated in polystyrene with use of thefollowing procedure. Ninety parts by weight of previously dried highimpact polystyrene pellets was blended with 0.1 part of mineral oil and10 parts by weight of dry NaBH₄ powder. The resultant mixture was thenextruded at about 450° F. and pelletized as described in Example 1. Asin Example 1, an excellent dispersion of minute particles of NaBH₄ inthe resin was produced with virtually no gas bubbles observed.

EXAMPLE 4

An activating agent concentrate was prepared by the following procedure.28.6 parts by weight of CAB-O-SIL fumed silica was intensively mixedwith 71 parts by weight of a solution (65% solids) of polyacrylic acidin water for five minutes with use of a Henschel mixer. The resultantmixture was a "dry" free flowing powder.

EXAMPLE 5

The procedure described in Example 4 was also used to prepare anactivator concentrate containing 71.4 parts by weight of water. Thewater was added slowly to 28.6 parts by weight of CAB-O-SIL fumed silicawhile mixing. The concentrate handled as a "dry" powder.

EXAMPLE 6

Polypropylene foamed fibers were prepared as follows. The series ofblends of polypropylene resin powder, sodium borohydride, and CAB-O-SILwhose compositions are shown in Table I were prepared. The polypropylenepowder was not dried prior to blending, but rather was blended in the"as-received" condition.

                  TABLE I                                                         ______________________________________                                        Polypropylene                                                                           99.89   99.75    99.5  99.95 99.92                                  NaBH.sub.4                                                                              0.1     0.25     0.5   0.05  0.075                                  CAB-O-SIL 0.001   0.0025   0.005 0.005 0.00075                                ______________________________________                                    

These mixtures of Table I were extruded through a 3/4 inch diameter dieat 430°-440° F. to form filaments with varying amounts of orientedfoamed cells. A 31/2:1 compression ratio was employed. The moisturepresent on the NaBH₄ and the resin was sufficient to activate therelease of hydrogen to foam the fibres as they passed through the meltphase in the extruder.

EXAMPLE 7

Foamed polypropylene structural parts were prepared by the followingprocedure. Blends of undried polypropylene resin powder, sodiumborohydride, powdered stearic acid and CAB-O-SIL were prepared in aHenschel Mixer. Stearic acid functioned as the activator and CAB-O-SILas the nucleating agent for bubble formation in this system. This systemwas limited to 1% stearic acid activator due to the limited solubilityof the acid in the solid resin and because the lubricity in the barrelof the extruder provided by the molten stearic acid causes slippage ofthe screw. Although good foams were prepared with good densityreduction, stearic acid and other fatty acids are less than idealactivators because of the above mentioned factors. In any event,structural foam parts of good quality were produced on a reciprocatingscrew L/D 16:1, 100 ton, 5 oz. injection molding machine equipped with acut off nozzle. The compositions shown in Table II were so produced. Allpercentages are based on the weight of polypropylene resin.

                  TABLE II                                                        ______________________________________                                        SBH  Fumed Silica Stearic Acid                                                                             % Wt. Reduction                                  %    %            %          of Part Due to Foam                              ______________________________________                                        0.06 0.1          1.0        20.2                                             0.13 0.1          1.0        27.3                                             0.25 0.1          1.0        34.4                                             0.5  0.1          1.0        37.3                                             1.0  0.1          1.0        33.7                                             ______________________________________                                    

EXAMPLE 8

Foamed polystyrene structural parts were prepared by the followingprocedure. Polystyrene resin powder, the sodium borohydride concentrateprepared in Example 3, and either the activator of Example 4 or Example5 were blended in the proportion shown in Table III. All percentages arebased upon the weight of the resin.

                  TABLE III                                                       ______________________________________                                                               % Conc. of                                             % Conc. of             Example 4                                              Example 3  % Conc. of  Polyacrylic                                            NaBH.sub.4 Chemical                                                                      Example 5   Acid      % Part                                       Blowing Agent                                                                            H.sub.2 O Activator                                                                       Activator Wt. Reduction                                ______________________________________                                        1          0.56                  16                                           1          0.28                  15.1                                         2          0.28                  18.4                                         4          2.24                  26.8                                         8          2.24                  25.5                                                                1.15      14.4                                         1          0.14                  12                                           4          0.56                  23.3                                         25         0.7                   25.6                                         ______________________________________                                    

Excellent quality foams having very fine uniform pore size were producedwith use of a structural foam injection machine. Density reductions ofabout 25% using only 4% of the blowing agent concentrate (0.4% NaBH₄)and 2.24% of the silica/water activator concentrate are unusually highand very desirable for this type of structural foam. Normally, withnitrogen injection to provide cells, density reductions of less than 20%are achieved. Moreover, the borohydride blown foam was unusually white,whereas the nitrogen blown foams have a slightly yellow cast. TheseActivator concentrates such as water, organic acids, alcohols, etc.adsorbed on colloidal silica serve as ideal activators and nucleatingagents for these systems because the activators can be premixed withblowing agent concentrates in the desired ratio with no reactionoccurring. This mixture can then be blended with the resin to be foamedin the desired ratio and fed into the foaming machine to produce thefoamed article.

EXAMPLE 9

Extruded foamed sheets of NORYL resin were manufactured by the followingtechnique. NORYL resin was blended with the sodium borohydride blowingagent concentrate of Example 3 and an organic acid according to thepercentages listed in Table IV. All percentages are based on the weightof NORYL resin. These mixtures were extruded with a 3/4 inch diameterextruder.

                  TABLE IV                                                        ______________________________________                                                                   Acid                                                                          Sodium                                                      %                 Boro-                                              Chemical Chemical          hydride                                            Blowing  Blowing  Acid     Mole                                               Agent    Agent    Activator                                                                              Ratio  Degree of Foam                              ______________________________________                                        Example 3                                                                              1.0      Oleic    4/1    Highly foamed                               Example 3                                                                              1.0      Octanoic 4/1    Highly foamed                               Neat NaBH.sub.4                                                                        0.1      Octanoic 4/1    Highly foamed                               Example 3                                                                              1.0      Stearic  4/1    Highly foamed                               Example 3                                                                              1.0      --       --     Very slight                                 ______________________________________                                    

It can be observed from this example that practically no foaming occurswhere the chemical blowing agent is used in the absence of an activator.Furthermore, the NaBH₄ concentrates are just as efficient as anequivalent amount of Neat NaBH₄ for blowing. Neat NaBH₄ suffers manyother deficiencies previously described which are overcome by theconcentrates of the invention.

I claim:
 1. A method of making a solid, substantially inert, and stablealkali metal borohydride blowing agent concentrate, comprising:a. dryblending from 0.5% to 20% by weight of dried alkali metal borohydridewith a dried inert resinous polymer in which reactive protonic hydrogenatoms are substantially absent to form a mixture thereof containing lessthan about 0.1% by weight adsorbed water; b. heating said mixture to atemperature above the melting point of said resin to form a moltenmixture; c. extruding said molten mixture to form a solid, stable andsubstantially inert concentrate; and d. forming said extruded and cooledconcentrate into pellets.
 2. The method of claim 1 wherein:said dryblended mixture contains from about 2% to 10% by weight of said alkalimetal borohydride.
 3. The method of claim 1 wherein:said alkali metalborohydride is a member selected from the group consisting of sodiumborohydride or potassium borohydride.
 4. The method of claim 1wherein:said inert resinous polymer is a member selected from the groupconsisting of polyolefins, polydienes, polystyrene, polyphenyleneoxide-styrene, polyacrylates, polyvinylchloride, or polyvinylacetate. 5.The method of claim 1 wherein:said alkali metal borohydride is sodiumborohydride.